Initialize a large, fixed-size array with non-Copy types

Question

I’m trying to initialize a fixed-size array of some nullable, non-copyable type, like an Option> for some kind of Thing. I’d

Answer 1

You could use the Default trait to initialize the array with default values:

let array: [Option<Box<Thing>>; SIZE] = Default::default();

See this playground for a working example.

Note that this will only work for arrays with up to 32 elements, because Default::default is only implemented for up to [T; 32]. See https://doc.rust-lang.org/std/default/trait.Default.html#impl-Default-98

Answer 2

I'm copying the answer by chris-morgan and adapting it to match the question better, to follow the recommendation by dbaupp downthread, and to match recent syntax changes:

use std::mem;
use std::ptr;

#[derive(Debug)]
struct Thing {
    number: usize,
}

macro_rules! make_array {
    ($n:expr, $constructor:expr) => {{
        let mut items: [_; $n] = mem::uninitialized();
        for (i, place) in items.iter_mut().enumerate() {
            ptr::write(place, $constructor(i));
        }
        items
    }}
}

const SIZE: usize = 50;

fn main() {
    let items = unsafe { make_array!(SIZE, |i| Box::new(Some(Thing { number: i }))) };
    println!("{:?}", &items[..]);
}

Note the need to use unsafe here: The problem is that if the constructor function panic!s, this would lead to undefined behavior.

Answer 3

This is the "keep it simple" answer: just type out all the values:

struct Thing;
const SIZE: usize = 5;

fn main() {
    let array: [Option<Box<Thing>>; SIZE] = [None, None, None, None, None];
}

You could also use a build script to generate this code for you. For an example of this, see:

• How to create a static string at compile time